When I bought my Treo 600, I definitely planned to use it as a mobile phone, Palm OS PDA, digital camera, and MP3 player, but I didn’t imagine I might be using it as a USB drive, too. Softick Card Export II allows you to mount the SD card in the Treo as if it were a USB drive. While you could always pop the card out, put it in an SD card reader, and connect the card reader to a USB port, it certainly is a lot easier to just connect the Treo using the USB sync cable you already have.
Right now, they claim support only for Windows 200/XP/Me, but they are working on OS X and Linux support. You don’t have to install any software on the desktop machine, but their software needs to know how to talk to the USB drivers for each OS. Hopefully, it won’t take them too long to sort out any minor differences in how the app needs to respond when talking to a Linux or OS X box. The beta version of Card Export II currently costs $15.